Q.How will you bring about the following conversions in not more than two steps?

(i) Propanone to Propene

Concept: Reduction followed by dehydration.

Steps:

1. Reduce propanone with NaBH4 or LiAlH4 to propan-2-ol. …

(i) Propanone to Propene

1. Reduce propanone to propan-2-ol using NaBH4 or LiAlH4. CH3COCH3 --NaBH4/H2O--> CH3CH(OH)CH3
2. Dehydrate the alcohol to propene using conc. H2SO4 at 443 K (acid-catalysed E1 elimination). …

Here is a clear, concept-first solution for each conversion, using named reactions and stepwise logic.

(i) Propanone to Propene

Method: Reduction followed by Dehydration (2 steps)

• Step 1 (Reduction): Reduce propanone ($\text{CH}_3\text{COCH}_3$) to propan-2-ol using NaBH$_4$ (or LiAlH$_4$ / catalytic hydrogenation).

• Step 2 (Dehydration): Heat propan-2-ol with concentrated $\text{H}_2\text{SO}_4$ at 170°C to eliminate water, forming propene.

Key concept: Ketone → Alcohol → Alkene (via acid-catalyzed elimination).

(ii) Benzoic acid to Benzaldehyde

Method: Rosenmund Reduction (1 step)

• Step 1: Convert benzoic acid to benzoyl chloride using SOCl$_2$ (or PCl$_5$).

• Step 2: Reduce benzoyl chloride to benzaldehyde using H$_2$ / Pd-BaSO$_4$ (poisoned catalyst, Rosenmund reduction).

Key concept: Acid chloride → Aldehyde (selective hydrogenation stops at aldehyde).

(iii) Ethanol to 3-Hydroxybutanal

Method: Aldol Condensation (1 step)

• Step 1: Oxidize ethanol to ethanal using acidified K$_2$Cr$_2$O$_7$ (or PCC).

• Step 2: Perform aldol condensation of ethanal with dilute NaOH at room temperature.

Key concept: Two molecules of aldehyde combine to form a β-hydroxy aldehyde.

(iv) Benzene to m-Nitroacetophenone

Method: Friedel-Crafts Acylation followed by Nitration (2 steps)

• Step 1 (Acylation): Treat benzene with acetyl chloride ($\text{CH}_3\text{COCl}$) and anhydrous AlCl$_3$ to form acetophenone.

• Step 2 (Nitration): Treat acetophenone with a nitrating mixture ($\text{conc. HNO}_3$ + $\text{conc. H}_2\text{SO}_4$). The –COCH$_3$ group is meta-directing, so the product is m-nitroacetophenone.

Key concept: Meta-directing acyl group controls the position of the second substituent.

(v) Benzaldehyde to Benzophenone

Method: Grignard Reaction followed by Oxidation (2 steps)

• Step 1 (Grignard addition): React benzaldehyde with phenylmagnesium bromide ($\text{C}_6\text{H}_5\text{MgBr}$), then hydrolyze to form diphenylmethanol.

• Step 2 (Oxidation): Oxidize diphenylmethanol to benzophenone using PCC (or acidified K$_2$Cr$_2$O$_7$).

Key concept: Aldehyde + Grignard → Secondary alcohol → Ketone.

(vi) Bromobenzene to 1-Phenylethanol

Method: Grignard Reaction with Ethanal (2 steps)

• Step 1 (Grignard formation): Treat bromobenzene with Mg in dry ether to form phenylmagnesium bromide.

• Step 2 (Addition to aldehyde): React the Grignard reagent with ethanal ($\text{CH}_3\text{CHO}$), then hydrolyze to get 1-phenylethanol.

Key concept: Aryl halide → Grignard → Secondary alcohol.

(vii) Benzaldehyde to 3-Phenylpropan-1-ol

Method: Aldol Condensation followed by Hydrogenation (2 steps)

• Step 1 (Crossed Aldol): React benzaldehyde with ethanal in dilute NaOH to form cinnamaldehyde (3-phenylprop-2-enal). $\text{C}_6\text{H}_5\text{CHO} + \text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \text{C}_6\text{H}_5\text{CH=CHCHO}$ …

Common Mistakes in Oxidation Reaction Conversions (CBSE/JEE)

Students often lose marks in these conversions due to reagent confusion, overlooking reaction conditions, and ignoring functional group priorities. Below is a mistake-by-mistake breakdown.

(i) Propanone → Propene

Correct route:

Propanone $\xrightarrow{\text{NaBH}_4}$ Propan-2-ol $\xrightarrow{\text{Conc. H}_2\text{SO}_4, \, \Delta}$ Propene

Common mistakes:

• ✗ Using KMnO₄ or K₂Cr₂O₇ (oxidising agents) — these will not reduce the ketone.
• ✗ Trying direct dehydration of propanone — ketones do not dehydrate like alcohols.
• ✗ Using LiAlH₄ instead of NaBH₄ — LiAlH₄ works but is less common in 2-step problems; NaBH₄ is safer and sufficient.

How to avoid:

Remember: Ketone → Alcohol (reduction) → Alkene (dehydration). Use NaBH₄ for reduction, then conc. H₂SO₄ with heat.

(ii) Benzoic acid → Benzaldehyde

Correct route:

Benzoic acid $\xrightarrow{\text{SOCl}_2}$ Benzoyl chloride $\xrightarrow{\text{H}_2, \, \text{Pd-BaSO}_4 \, (\text{Rosenmund reduction})}$ Benzaldehyde

Common mistakes:

• ✗ Using LiAlH₄ — this reduces benzoic acid all the way to benzyl alcohol, not benzaldehyde.
• ✗ Using NaBH₄ — NaBH₄ does not reduce carboxylic acids.
• ✗ Forgetting to convert to acid chloride first — direct reduction of –COOH to –CHO is not possible in one step.

How to avoid:

Always convert –COOH to –COCl first, then use Rosenmund reduction (H₂/Pd-BaSO₄). Never use strong reducing agents like LiAlH₄ here.

(iii) Ethanol → 3-Hydroxybutanal

Correct route:

Ethanol $\xrightarrow{\text{Cu, \, 573 K}}$ Ethanal $\xrightarrow{\text{Dil. NaOH, \, \text{aldol condensation}}}$ 3-Hydroxybutanal

Common mistakes:

• ✗ Using conc. NaOH — this leads to dehydration of the aldol product, giving crotonaldehyde instead.
• ✗ Trying to oxidise ethanol directly to 3-hydroxybutanal — impossible; you need two separate steps.
• ✗ Using KMnO₄ for oxidation — this overoxidises ethanol to acetic acid.

How to avoid:

First step: controlled oxidation (Cu/573 K or PCC) to get ethanal. Second step: aldol condensation with dilute NaOH at low temperature.

(iv) Benzene → m-Nitroacetophenone

Correct route:

Benzene $\xrightarrow{\text{CH}_3\text{COCl, \, AlCl}_3}$ Acetophenone $\xrightarrow{\text{Conc. HNO}_3 + \text{Conc. H}_2\text{SO}_4}$ m-Nitroacetophenone

Common mistakes:

• ✗ Nitrating before acylating — nitrobenzene is deactivated and cannot undergo Friedel-Crafts acylation.
• ✗ Using CH₃Cl + AlCl₃ (alkylation) — this gives toluene, then nitration gives o/p-nitrotoluene, not m-nitroacetophenone.
• ✗ Forgetting that –COCH₃ is meta-directing — students sometimes expect ortho/para products.

How to avoid:

Always do Friedel-Crafts acylation first, then nitration. The acyl group deactivates the ring and directs meta.

(v) Benzaldehyde → Benzophenone

Correct route:

Benzaldehyde \xrightarrow{\text{C}_6\text{H}_5\text{MgBr, \, \text{then H}_3\text{O}^+}} 1,2-Diphenylethanol $\xrightarrow{\text{PCC or CrO}_3}$ Benzophenone

Common mistakes:

• ✗ Using C₆H₅MgBr directly on benzaldehyde without protecting the aldehyde — Grignard reagent reacts with –CHO first.
• ✗ Trying Friedel-Crafts acylation with benzoyl chloride on benzene — that gives benzophenone, but starting from benzaldehyde requires a different route.
• ✗ Forgetting the oxidation step after Grignard addition.

How to avoid:

Add phenylmagnesium bromide to benzaldehyde (nucleophilic addition), then oxidise the secondary alcohol to ketone using PCC or CrO₃.

(vi) Bromobenzene → 1-Phenylethanol

Correct route:

Bromobenzene $\xrightarrow{\text{Mg, dry ether}}$ C₆H₅MgBr $\xrightarrow{\text{CH}_3\text{CHO, then H}_3\text{O}^+}$ 1-Phenylethanol

Common mistakes:

• ✗ Using CH₃COCl instead of CH₃CHO — this gives acetophenone, not the alcohol.
• ✗ Forgetting dry ether — Grignard formation fails in presence of moisture.
• ✗ Trying Wurtz-Fittig reaction — that gives alkylbenzene, not alcohol.

How to avoid:

Make Grignard reagent from bromobenzene, then react with ethanal (acetaldehyde), followed by hydrolysis.

(vii) Benzaldehyde → 3-Phenylpropan-1-ol

Correct route:

Benzaldehyde $\xrightarrow{\text{CH}_3\text{CH}_2\text{MgBr, then H}_3\text{O}^+}$ 1-Phenylpropan-1-ol $\xrightarrow{\text{Red P + HI or H}_2/\text{Pd}}$ 3-Phenylpropan-1-ol

Common mistakes:

• ✗ Using CH₃MgBr — this gives 1-phenylethanol (only 2 carbons added).
• ✗ Forgetting the reduction step — Grignard addition gives a secondary alcohol, not the primary alcohol required.
• ✗ Using LiAlH₄ on benzaldehyde directly — that gives benzyl alcohol, not 3-phenylpropan-1-ol.

How to avoid:

Use ethylmagnesium bromide (C₂H₅MgBr) to add two carbons, then reduce the –OH to –H using Red P/HI or catalytic hydrogenation.

(viii) Benzaldehyde → α-Hydroxyphenylacetic acid

Correct route: …